Radiation curing is a chemical manufacturing process which utilizes electromagnetic radiation energy to change the chemical and physical nature of organic chemical materials by cross-linking, polymerizing, drying, solidifying or degrading polymer network. Radiation curing has been applied to a variety of materials including paper, wood, metal, leather, vinyl, plastic, glass, magnetic recording tape, human hair, ink, etc. Radiation curing processes are commercially applied today for adhesives (such as pressure sensitive tapes and labels, laminated foils and films, flocked materials for automotive and shoe applications, structural adhesives, etc.), abrasives (such as coated abrasive products for microfinishing, glass lens grinding and polishing, burnishing, etc.), release coatings (such as silicone coatings on paper and plastics), metal coatings, overprint coatings, flooring, wood finishing, food and perfume packaging, printing, photopolymer plates, electronics, dental and medical applications, magnetic media, fiber optics, lithography, etc.
The primary sources of radiation for radiation curing include electron beam (e-beam), infrared (IR), ultraviolet (UV), and laser.
It is generally known that a part of radiation energy used for curing resins and coatings is converted to heat during curing, causing an increase in the atmosphere temperature. It is also known that an increase in the atmosphere temperature will raise the temperature of the coating being cured, thereby damaging the coating or resulting in cured coating with undesirable properties. Therefore, it is desirable to carefully control the temperature of both the atmosphere and the coating being cured to obtain the desired material properties.
Overheating of the atmosphere and the coating being cured can be controlled by using a high speed at which the curing coating traverses under the radiation source. Additionally, a water-cooled drum positioned directly under the radiation source can also be used to help in removing heat from the radiation chamber and the substrate being cured. However, black bodies, soft substrates, and temperature sensitive substrates still require proper care and an external source for removing heat from the radiation chamber. In order to properly control the temperature in the radiation chamber and produce cured coatings with desirable properties, these materials are generally processed in the presence of high flow rates of an ambient temperature and high purity (containing 99.999% by volume inert gas) inert gas, such as nitrogen, argon or helium. The high flow rate of high purity inert gas helps in removing heat and preventing buildup of heat in the radiation chamber.
Besides controlling the temperature of the coating, high purity inert gas prevents buildup of oxygen in the radiation curing chamber, which is known to abstract hydrogen from the coating material and inhibit the curing process, as well as form ozone gas. The effect of oxygen on performance of sources of radiation and applications of radiation curing can be found in "Radiation Curing Primer I: Inks, Coatings and Adhesives" published by RadTech International North America, Northbrook, Ill. (1990) Page 51. The presence or buildup of oxygen in the radiation curing chamber is not desirable because it will be converted to ozone in the presence of a radiation source, presenting safety and health related problems and requiring additional processing steps to treat chamber gases prior to their venting or disposal.
The use of high purity nitrogen gas for inerting radiation curing chambers and purging oxygen barriers has been known for years. It has been disclosed in U.S. Pat. Nos. 4,252,413 and 4,303,695. The nitrogen gas used in radiation curing is generally supplied by vaporizing more expensive cryogenically produced high purity liquid nitrogen.
The production of high purity nitrogen has, for many years, been carried out by employing state-of-the-art air separation technology based on cryogenic distillation techniques. Because of the favorable economics of scale-up for such cryogenic distillation, large tonnage nitrogen users are supplied with nitrogen gas piped from a cryogenic plant installed on the users' site. Smaller tonnage users, i.e., 2-30 tons/day or less, are typically supplied with liquid nitrogen trucked to the users' site from a centrally located liquid nitrogen production plant. The cost of liquefying nitrogen gas and of transporting the liquid nitrogen from an off-site cryogenic plant to the users' site adds significantly to the cost of the nitrogen as supplied to the user.
In recent years, therefore, a major challenge in the art has been to develop small tonnage air separation plants that can effectively produce low cost nitrogen gas at the users' site. Recent developments relating to nitrogen production by noncryogenic air separation technologies such as pressure swing adsorption (PSA) and membrane technologies have served to significantly lower the cost of on-site systems for the production of low purity, small tonnage nitrogen. On the other hand, high purity nitrogen cannot be economically produced by such PSA or membrane systems because of practical imitations rendering the power requirements and the cost of such systems prohibitive.
A hybrid atmosphere radiation curing system utilizing inexpensive, relatively impure, non-cryogenically generated nitrogen containing up to 5% by volume residual oxygen as an impurity for purging oxygen barriers and high purity, more expensive, cryogenically generated pure nitrogen stream containing less than 10 ppm by volume residual oxygen for purging the processing chamber has been disclosed in U.S. Pat. No. 5,120,972. According to the teachings of this patent, the overall atmosphere cost can be substantially reduced by using an impure, low cost non-cryogenically generated nitrogen for purging oxygen barriers. More importantly, this patent teaches against using non-cryogenically generated nitrogen in the radiation curing chamber.
U.S. Pat. No. 4,985,274 discloses a process of purifying nitrogen containing impurities in the form of oxygen by reacting it with a reducing gas in the presence of a radiation source. The purification of a nitrogen stream containing oxygen with a radiation source is neither desirable nor recommended because of the formation of ozone and subsequent safety problems caused by the formation of ozone. OSHA limits workers exposure to less than 0.1 ppm TWA for an 8 hour exposure.
Various forms of cryogenic and non-cryogenic nitrogen production and purification are known as set forth in the art below.
U.S. Pat. No. 5,154,892 discloses a method and apparatus for maintaining an inert atmosphere at an irradiation chamber using pure inert gas such as nitrogen, as well as recirculated inert gas.
U.S. Pat. No. 3,535,074 discloses the conversion of oxygen present in nitrogen to water by reaction with hydrogen in the presence of a noble metal catalyst.
U.S. Pat. No. 4,931,070 discusses the separation of air by membrane to produce nitrogen in which residual oxygen is reacted to create water. The wet nitrogen is dried by membrane separation on.
U.S. Pat. No. 5,004,482 discloses a process for producing dry nitrogen from a pressure swing adsorptive separation of air with membrane drying either before or after the air separation.
U.S. Pat. No. 5,077,029 discloses the controlled introduction of hydrogen into an oxygen containing nitrogen product of membrane or adsorptive air separation.
U.S. Pat. No. 5,122,355 discloses membrane separation of air to produce nitrogen which is further treated to reaction with hydrogen over a catalyst to convert oxygen to water with subsequent adsorptive drying.
U.S. Pat. No. 4,859,435 discloses the deoxygenation of nitrogen by catalytic reaction with methanol.
U.S. Pat. No. 4,249,915 describes adsorptive removal of moisture and carbon dioxide from air in separate adsorptive beds.
Other patents of interest are U.S. Pat. No. 4,954,144 and 4,994,095.
In order to lower the costs of nitrogen used for inert gas radiation curing, the art has suggested to use high purity cryogenically produced nitrogen at the radiation chamber and lower purity non-cryogenically produced nitrogen at the outer gaseous knives of radiation curing apparatus. The disadvantage of this hybrid system is that two nitrogen sources are required and expensive liquid nitrogen is still required for purging oxygen from the radiation chamber. This liquid nitrogen, at the capacities required, cannot justify on-site generation, but must be trucked in from off-site cryogenic air separation plants at considerable expense and subject to potential inconsistent delivery. The present invention overcomes the drawbacks of this prior art by providing on-site generation of non-cryogenic nitrogen at purity requirements sufficient for a radiation chamber of a radiation curing apparatus which provides lower cost inerting with consistent supply, as set forth in greater detail below.